1. Field of the Disclosure
The present disclosure relates to methods for obtaining a resistivity parameter of a formation from a logging tool in a borehole penetrating the formation. In particular, the present disclosure relates to correcting resistivity measurements for the effect of tool eccentricity within the borehole.
2. Description of the Related Art
Multiple Propagation Resistivity (MPR) tools are often used for measuring resistivity properties of a formation. An MPR tool typically includes transmitter coils and receiver coils having magnetic moments oriented parallel to and coincident with the tool axis. Increasingly, MPR tools are used for the purpose of determining well placement and to navigate reservoirs so as to maintain a borehole trajectory along a predetermined path. For example, it may be desired to place the borehole in an oil-producing formation for most of the borehole trajectory. However, MPR tools do not detect azimuthal variations in resistivity. An azimuthal propagation resistivity (APR) tool has recently been introduced that is sensitive to azimuthal variations in resistivity. The APR tool typically includes transmitter coils as well as receiver coils that are oriented perpendicular to the transmitter coils. As a result of azimuthal sensitivity, an APR tool can be responsive to formations ahead of the APR tool. Directional information may thereby be provided regarding formations ahead of the tool.
Signal responses obtained at an APR tool are generally affected by the degree of eccentricity of the APR tool from the central axis of the borehole. In horizontal and highly-deviated boreholes, tool eccentricity is common since the tool may sag toward or even rest upon a lower portion of a horizontal borehole due to the pull of gravity. The APR tool signal may thus be a superposition of a signal obtained from a nearby formation and a signal resulting from the eccentricity of the APR tool. An additional issue is related to the invasion of borehole mud into the formation. An invasion front of the borehole mud, which may be reasonably assumed to be azimuthally symmetric when occurring in a vertical borehole, is more likely to be azimuthally asymmetric when the occurring in a deviated or horizontal borehole. In other words, the invasion front may extend further below the borehole than above, due to the pull of gravity. These asymmetrical effects induce an additional unwanted signal at the APR tool.
APR tools are typically designed to minimize these tool eccentricity effects by, for example, using a bucking coil. In general, the bucking coil works effectively in canceling a field that is directly coupled from the transmitter coil to the receiver coils and helps reduce tool eccentricity effects and other near-borehole effects. However, in some cases tool eccentricity effects may still be significant. Additional effects may be related to the resistivity of the formation or of the mud used in drilling. For example, in a borehole having a resistive drilling mud therein and which penetrates a shale bed, the contribution of unwanted signals to the APR tool measurements can be anomalously large. Modeling results confirm that measurements obtained from a borehole filled with resistive mud may display a larger eccentricity effect than measurements obtained from a borehole filled with conductive mud. Therefore, there is a need to remove or reduce the effects of eccentricity on the APR measurements.